This invention relates to a method for preparing a C/Si/O composite material having oxidation resistance and useful as heat resistant materials, heating materials, structural materials and lithium ion secondary cell negative electrode materials.
On account of prominent characteristics including good electrical conduction, high-temperature strength, self lubricity, chemical stability and workability, carbon is an excellent high-temperature material which finds general use as metal melting crucibles, steel making electrodes, electrochemical electrodes, heaters, structural materials and lithium ion secondary cell negative electrode materials. However, carbon can be burn out in an oxidizing atmosphere at high temperatures, which problem restricts the application of carbon.
Recently, carbon-inorganic composite materials have been developed in order to improve the oxidation resistance of carbon without impairing the function of carbon itself.
Exemplary carbon-inorganic composite materials include inorganic coated carbon materials in which inorganic material such as SiO2 or SiC is coated, SiC/carbon composite materials in which carbon-impregnated SiO2 glass is reacted and converted to SiC, and carbon/ceramics composite materials obtained by mixing carbon sources with inorganic materials, such as carbon/B4C and carbon/B4C/SiC.
It is true that the composite materials obtained by combining carbon with inorganic materials according to the above-mentioned methods are improved in oxidation resistance that carbon lacks. However, since they are composite materials with SiC based ceramics, their production requires reaction at relatively high temperatures, and the conditions under which SiC is formed must be carefully controlled. As a consequence, production on an industrial scale is difficult and the production cost increases, resulting in expensive products.
An object of the invention is to provide a method for preparing a C/Si/O composite material having oxidation resistance at a low cost.
It has been found that by impregnating graphite, which is known to selectively adsorb oily components, with a crosslinkable organosilicon compound and using the impregnated graphite as a precursor, rather than forming composite materials of carbon with SiC based ceramics as in the prior art, there is obtained a C/Si/O composite material having excellent oxidation resistance.
In a first aspect, the invention provides a method for preparing a C/Si/O composite material, comprising the steps of impregnating graphite with at least one organosilicon compound selected from crosslinkable silanes and siloxanes; causing the organosilicon compound to crosslink within the graphite; and heating the graphite at a temperature of 300 to 1,200xc2x0 C. in a non-oxidizing gas for reaction to take place.
In a second aspect, the invention provides a method for preparing a C/Si/O composite material, comprising the step of further heat treating the C/Si/O composite material obtained by the method of the first aspect at a temperature of 800 to 1,600xc2x0 C.
In the method for preparing a C/Si/O composite material according to the first or second aspect, the crosslinkable organosilicon compound is preferably an addition reaction curing organopolysiloxane composition or a curable silicone resin. The graphite is most often expanded graphite.